A standard self-tightening hammer-drill chuck that is rotated about an axis of a drill spindle to rotate a drill bit about and reciprocate it along the axis has a chuck body securable to the spindle and formed centered on the axis with a screwthread. A sleeve rotatable but axially nondisplaceable on the chuck body is formed angularly equispaced about the axis with a plurality of angled jaw guides, although it is possible to form the guides on the chuck body and the screwthread on the sleeve for the same effect. Respective jaws in the guides have racks that mesh with the screwthread so that rotation of the sleeve body on the chuck body about the axis in a tightening direction moves the jaws radially together and opposite rotation in a loosening direction moves the jaws radially apart. The normal clockwise (seen from behind the tool) rotation of the drill chuck creates reaction forces in the chuck which tend to rotate the sleeve in the tightening direction for automatic self-tightening action.
It is also standard to provide such a chuck with a locking ring that is limitedly angularly displaceable relative to the chuck body. In addition this ring is formed with teeth or other formations that mesh axially with complementary formations on the sleeve and the ring is axially displaceable on the chuck body between a back unlocked position with the formations disengaged and relative rotation of the ring and sleeve possible, and a front position with the formations in mesh and such relative rotation impossible. The function of this ring is to prevent accidental opening of the chuck. The limited angular movement of the ring on the chuck body is necessary to allow the above-described self-tightening action to take place and is created by providing the ring or chuck body with a pin projecting radially into a slot on the other body that is of somewhat greater angular width than the pin.
It is possible as described in U.S. Pat. No. 4,836,563 for the jaws to have teeth directly meshing with a screwhead on the chuck body, or as described in German patent document 3,432,918 the jaws can be guided externally on the sleeve on pushers that themselves are moved by a screwthread. In both cases the locking ring is held by a spring or snap fit in its two axially offset end positions.
With such an arrangement when a drill bit is to be chucked the locking ring is first pulled back, and then the sleeve is rotated in the unlocking direction, normally counter-clockwise as seen from axially behind the chuck, to spread the jaws sufficiently to accept the tool. Then the sleeve is rotated in the locking direction until the jaws snugly engage the shank of the tool, and finally the locking ring is pushed back forward so that its teeth engage those of the sleeve and impede it from rotating substantially on the chuck body.
Thus with the standard system it is entirely possible for the locking ring to be in its angular end position corresponding to engagement of its formation in the locking direction with the chuck body or vice versa, that is in such a position that it completely impedes further tightening of the chuck. When the locking ring is allowed to move back forward into this position after a new tool is fitted to the chuck, the result is that the subsequent automatic self-tightening action is prevented by the locking ring. To avoid this the user must remember before resetting the locking ring to rotate it in the unlocking direction to the extent of its angular travel so that when pushed forward there is still some room for automatic self-tightening of the chuck.
This problem is particularly evident in the system of above-mentioned German No. 3,432,918 when a spring is provided that continuously urges the locking ring in the tightening direction. Even when this spring is not used, as in above-mentioned U.S. Pat. No. 4,836,563, and instead a spring is provided that is effective oppositely, when the force of this spring is overcome as the locking ring is pushed forward it is possible to set it ahead in the locking direction, limiting subsequent self-tightening as with the older prior-art systems. In addition such a system does not work well with small forces, as for instance with a small diameter drill bit.